The gpt.vmoptions file affects only gpt. For configuring snappy
you need to the following:
You need to change the configuration in the snappy.ini file. The file is located in the snappy folder.
Change the line
# java_max_mem: 4G
to e.g.
java_max_mem: 6G
This means that snappy can use 6GB of you RAM. A recommended value is 70%-80% of the available RAM in your system.